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A 2500-year climate and environmental record inferred from subfossil chironomids from
Lugu Lake, southwestern China
Jie Chang1,2, Enlou Zhang1*, Enfeng Liu1,3, Weiwei Sun1, Pete G. Langdon4, James Shulmeister2
Affiliation:
1. State Key Laboratory of Lake Science and Environment, Nanjing Institute of
Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road,
Nanjing 210008, Jiangsu, China. China
2. School of Earth and Environmental Sciences, The University of Queensland, Australia
3. College of Geography and Environment, Shandong Normal University, China
4. Geography and Environment, University of Southampton, Southampton, UK
*Corresponding author: [email protected]
Key Words:
Subfossil chironomids, southwestern China, catchment erosion, sediment geochemistry,
monsoon precipitation, human impact
Abstract
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We present a sediment record from Lugu Lake, a large and deep alpine lake in southwestern
China, spanning the last c.2500 cal yr BP. This multi-proxy study focussed on subfossil
chironomids but also included analyses of chemical elements using inductively coupled
plasma-atomic emission spectrometry (ICP-AES), magnetic susceptibility, total organic carbon
(TOC), total nitrogen (TN) and grain size. The chironomid assemblage is dominated by
Procladius, Tanytarsus gracilentus-type and Polypedilum nubeculosum-type throughout the
core. The record reveals that these chironomid taxa responded to two periods of catchment
erosion between c.1700-1100 cal yr BP and c.1920 A.D. to the present. We provide evidence
which suggests that the recent erosion episode is caused by human activities; however, the
earlier event (c.1700-1100 cal yr BP) is likely related to increased regional precipitation,
possibly linked to the strengthening of the Indian summer monsoon in the late Holocene. It is
notable that the chironomids, through their varied ecologies, are able to detect the human
induced changes as well as natural climate changes, for instance, enhanced precipitation.
Coupling palaeoecological studies using chironomids with more traditional catchment erosion
indicators is thus a powerful tool for reconstructing past environmental and climate change.
Introduction
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Reconstructing past regional precipitation variability is a key to understanding the evolution
and impact of global climate systems such as the Asian summer monsoon from sub-decadal to
multi-millennial time scales. In the past decades, numerous paleoclimatic records have been
derived based on a diverse range of natural archives for these purposes, including but not
limited to stalagmites, marine, lake and peatbog sediments and tree-rings (Morrill et al. 2003;
Overpeck et al. 1996; Wang et al. 2010a). The Yunnan Plateau (23-28°N, 97-105°E, Fig. 1a),
southwest China, lies in the southern extension of the Tibetan Plateau and contains a large
number of high altitude mountain lakes. A few of these lakes have continuous organic records
which extend back to c.50 ka. Some of these lake sediment records have been reviewed in
Hodell et al. (1999), Overpeck et al. (1996), Wang et al. (2010b) and Cook and Jones (2012).
Recently, more high-resolution and absolute-dated records have emerged from the northern and
central part of Yunnan Plateau (Chen et al. 2014a; Chen et al. 2014b; Chen et al. 2014c; Cook
et al. 2011; Wang et al. 2017; Wang et al. 2014; Xiao et al. 2014; Zhang et al. 2017a; Zhang et
al. 2016; Zhang et al. 2017c) (Fig. 1a). These studies have improved our understanding of the
long-term relationship of the regional precipitation change and the Indian Ocean Summer
Monsoon over a multi-millennial time scale in southwestern China. There is however, still a
dearth of studies that focus on developing proxies that can be used for separating local and
regional events. This is critical to better interpret the regional records in the context of the
Asian monsoon variability and to avoid confusion with local environmental impacts.
To achieve a more comprehensive understanding of the response of lake sediment archives to
local and regional forcing, a suite of proxies are required to disentangle potentially complex
histories. In this study, we use chironomids (Diptera: Chironomidae), non-biting midges, which
are widely distributed in all permanent and semi-permanent terrestrial water bodies (Armitage
et al. 1995). The species distribution and life cycles of chironomid larvae are controlled by a
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range of limnic conditions (Brooks et al. 2007; Walker 1995). Consequently, quantitative
transfer functions based on chironomid taxon assemblages have been developed for
reconstructing a range of parameters, including summer air temperatures (Chang et al. 2015;
Heiri et al. 2011; Lotter et al. 1999), lake trophic status (Brooks et al. 2001; Langdon et al.
2006; Woodward and Shulmeister 2006; Zhang et al. 2006), water depth (Cwynar et al. 2012;
Engels and Cwynar 2011), pH (Rees and Cwynar 2010) and salinity (Zhang et al. 2007).
Recently, a chironomid-based temperature transfer function has been developed and applied for
SW China (Chang et al. 2017; Zhang et al. 2017a; Zhang et al. 2017b). These studies have
advanced our knowledge on the chironomid distribution and the controlling environmental
variables in the region. In addition, chironomids have also been demonstrated to be effective
indicators for human-induced impact. For instance, they respond to changes of substrate and
nutrient status that were induced by soil erosion. Because this response is indirect, a multi-
proxy approach is often employed in such studies notably from Northern Europe, east Africa
and southern China (Eggermont and Verschuren 2003; Potito et al. 2014; Taylor et al. 2017;
Zhang et al. 2011; Zhang et al. 2012). Occasionally, they have also been applied as a proxy to
indirectly reconstruct regional precipitation (e.g. in northern China (Wang et al. 2016) and
southern Chile (Massaferro and Brooks 2002)).
This study presents the record of chironomids, grain size and geochemical analyses from Lugu
Lake sediment covering the last 2500 years. Previous work from Lugu Lake had focused on
chironomids from more contemporary sediments (Guo et al. 2013; Zhang et al. 2013b), lipid
biomarkers (Li et al. 2017), diatoms, C:N ratio and pollen-based climate reconstructions (Wang
et al. 2017; Wang et al. 2014). Here, we used multi-variate statistical analyses to study the
chironomid responses to local environmental changes at the catchment level. We then
compared this record to regional paleoclimatic reconstructions to interpret the possible links to
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human-induced environmental impact and natural climate variability such as regional
precipitation changes.
Materials and methods
Study site
Lugu Lake (27°41–27°45N, 100°45–100°50E, 2690 m a.s.l.) is a natural and relatively large
alpine lake in Yunnan Province, southwestern China (Fig. 1a). The lake has a semi-closed
basin with a maximum water depth of 93.5 m and a mean depth of 40.3 m. It has a surface area
of 48.45 km2, and a catchment drainage area of 171.4 km2 (Fig. 1b). Lugu Lake is located in
the catchment of the Yalong River (a branch of the Yangtze River), and the modern catchment
characteristics, water quality measurements and vegetation composition have been summarized
in a few past studies (Guo et al. 2013; Wang et al. 2014; Zhang et al. 2013a; Zhang et al.
2013b). In general, the lake has been impacted by widespread logging between the 1950s and
1980s, followed by tourism impact in the past 20 years. Lugu Lake is located in a region which
is under strong influence of Indian Summer Monsoon. The climate in this area is atypical sub-
tropical due to its high elevation despite the sub-tropical latitude (27 °S). Based on
interpolating climate data from regional stations, the mean annual air temperature at Lugu Lake
is approximately 12.8°C and the annual precipitation is approximately 1000 mm (Böhner 1994;
Böhner 2006). The area has distinct dry and wet seasons where 80–90% of precipitation occurs
between May and October each year.
A 95-cm sediment core was recovered at the water depth of 64 m, close to the centre of Lugu
Lake in July, 2007 (Fig. 1b). This was achieved using an UWITEC gravity corer. The sediment
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core was sub-sampled at 0.5 cm contiguous intervals and packaged onsite. The sub-samples
have been stored in a refrigerator (at 4 °C) until laboratory analysis.
Sediment core chronology of Lugu Lake
Freeze-dried and powdered sediment samples were weighed and prepared for dating purposes.
Caesium-137 (137Cs) activity was measured by emissions at 662 keV using an Ortec HPGe
GWL series well-type coaxial low background intrinsic germanium detector from the top 15
cm. Six samples of fossil plant fragments were taken from the depths of 18, 30, 42, 60, 85 and
92 cm for Accelerator Mass Spectrometry (AMS) radiocarbon dating at the Rafter Radiocarbon
Laboratory at the Institute of Geological and Nuclear Sciences, New Zealand. The AMS 14C
dates were calibrated to calendar years before present (0 cal. yr BP = AD 1950) using CALIB
7.1 with IntCal 13 calibration dataset (Reimer et al. 2013). 137Cs and the AMS 14C dating results
were used in combination to produce the final chronological model in the Clam 2.2 software
(Blaauw, 2010) (Fig. 1c).
Chironomid sample analysis
All sediment pre-treatment, sample preparation and sample analyses were performed in the
laboratories at the State Key Laboratory of Lake Science and Environment, Nanjing Institute of
Geography and Limnology (NIGLAS). Chironomid samples were prepared at every 4 cm
interval throughout the 95 cm sediment core using 1 cubic centimetre of sediment, following
the protocol described in Brooks et al. (2007). Sediment samples were deflocculated in a warm
(c.70 °C) water bath with 10% solution of potassium hydroxide (KOH) for 20 minutes and
subsequently sieved with a 90 µm mesh-size sieve. Sieved residues were stored in distilled
water and chironomid head capsules were hand-picked under a dissection microscope at 50 ×
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magnification using fine forceps. All head capsules were mounted in a solution of Hydromatrix
after a minimum of 50 counts was reached. The chironomid species were then identified
following regional and northern hemisphere diagnostic keys (Andersen et al. 2013; Brooks et
al. 2007; Oliver 1981; Tang 2006; Wiederholm 1983).
Sediment multi-proxy analyses
Samples were prepared for magnetic susceptibility (MS), grain size, metal elements, total
organic carbon (TOC) and total nitrogen (TN) analyses where sub-samples were taken at every
0.5 cm interval for each analysis. For MS analysis, about 0.1 g of freeze-dried samples were
placed in an 8 ml polystyrene pot that had been acid washed. The MS values (χlf) were then
measured at 0.47 kHz using a Bartington MS2 Meter for each sample (Dearing 1994; Oldfield
and Richardson 1990). Grain size distribution of the samples was measured using a Malvern
Instruments Mastersizer-2000. The grain size preparation method involved 1 g of freeze-dried
sediment samples being pre-treated with c. 20 ml of 5% H2O2 to remove organic matter and
then with 10 ml of 10% HCl to remove carbonates. Deionized water was added and kept for 24
hours to wash off acidic ions. Approximately 10 ml of 0.05 M (NaPO3)6 was subsequently
added to the treated samples and they were placed in an ultrasonic bath for 15 min before
measuring grain size. Approximately 0.1 g of freeze-dried and ground samples were
completely digested with HF-HCl-HNO3-HClO4 and prepared for the determination of metal
elements (Al and Ti). Elemental concentrations were measured using inductively coupled
plasma-atomic emission spectrometry (ICP-AES) (Liu et al. 2012; McQuaker et al. 1979). A
similar amount (c. 0.1 g) of freeze-dried and homogenized samples were pre-treated with 10%
HCl to remove carbonate, and subsequently freeze-dried and ground for TOC and TN analyses.
Both measurements were conducted by combusting the samples using a Euro EA3000
Elemental Analyzer.
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Statistical methods
Several ordination analyses were performed using the CANOCO version 4.5 program package
(ter Braak and Šmilauer 2002) to explore the changes in subfossil chironomid species
assemblages in Lugu Lake and to determine which variable(s) (MS, grain size fraction < 4, Al
and Ti, TOC, TN) explained a significant proportion of the temporal variation in the
chironomid assemblages. We focus on the clay and fine silt fraction of the grain sizes because
it shows the most variation of all fractions. We selected Al and Ti in this study because both of
these elements are critical components of the continental soil in southwestern China (Ji et al.
2004), though it is likely that they show similar pattern. MS and grain sizes, TOC and TN of
the sediment also vary similarly. However, they were all selected for the multi-variate analyses
with the chironomid data because we aim to test which of these variables have a more direct
and stronger influence on chironomid species variation through time.
Geochemical proxy data were log10 (x+1) transformed, and chironomid and grain size data
were square-root transformed prior to the ordination analyses. Detrended Correspondence
Analysis (DCA) (Hill and Gauch 1980) with detrending by segments was applied to explore
the major temporal patterns of chironomid assemblage variation and to determine the gradient
lengths. The result showed that the gradient length of DCA axis 1 was 1.12 standard deviation
(SD) suggesting that either a linear or unimodal approach could be applied (ter Braak and
Šmilauer, 2002).
We then performed a series of Redundancy Analysis (RDAs) using a manual selection option
by including six parameters (MS, grain sizes fraction < 4 µm, Al, Ti, TOC and TN) as
explanatory variables to identify which ones explained significant variance in the chironomid
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species (with species data square-root transformed and down-weighting of rare species) and the
distribution of chironomid samples along the ordination gradients. The significance of each
variable was tested using the Monte Carlo permutation tests (999 unrestricted permutations).
The significance level was adjusted by applying a ‘One Step’ Bonferroni correction (García
2004).
Results
Chronology
The 137Cs dating results show that at 9 cm depth, there is a peak in 137Cs activity. This is
assumed to correspond with the 1963–1964 maximum fallout in the atmosphere from nuclear
bomb testing (Appleby 2001). The age-depth model was therefore developed based on a 137Cs
date of 1963 AD and six calibrated AMS 14C dates by fitting a smooth-spline model (Blaauw
2010) (Fig. 1c). The 95-cm core has a basal age of c.2500 cal yr BP and the sedimentation rate
has an average of c. 2.5 cm per 100 years and a rapid increase is observed for the last c.200
years (Fig. 1c).
Chironomid taxa in Lugu Lake and other proxies
Thirteen non-rare chironomid taxa (i.e. occurred in more than 2 samples and with an overall
abundance of larger than 2%, i.e. N and N2 > 2) were identified in this 2500-year Lugu Lake
core (Fig. 2). All samples yield a total count of over 50 chironomid head capsules with a
maximum count of 276 reached at the depth of 21 cm (c.260 cal yr BP). The chironomid
assemblage is dominated by Procladius, Tanytarsus gracilentus-type and Polypedilum
nubeculosum-type throughout the record. These three taxa reached a maximum abundance of
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86% (c.1900 cal yr BP), 25% and 34% (both at c.1300 cal yr BP), respectively. A few other
taxa such as Chironomus plumosus-type, Dicrotendipes nervosus-type and Harnischia are
abundant between c.1900 and c.1100 cal yr BP (Fig. 2). Based on changes in chironomid
stratigraphy, the record is divided into three zones (Table 1) using the cluster analysis (Grimm
1987). The results of magnetic susceptibility, the content of clay and fine silt (grain size
fraction < 4 µm) and sand (grain size fraction > 63 µm), concentrations of Ti and Al, TOC and
TN are presented in Fig 3a-g and also described in Table 1.
Ordination results
The DCA analysis performed on the chironomid data show that the first two DCA axes
explained 16.9 and 10.4% of the chironomid assemblage variance respectively. The DCA axis
1 sample score (Fig. 2) shows high values between 2500 and 1700 cal yr BP (chironomid zone
1) and in the last 1100 years (chironomid zone 3) while low values prevail between 1700 and
1100 cal yr BP (chironomid zone 2). Changes in abundance of two chironomid taxa:
Procladius and Tanytarsus gracilentus-type are significantly correlated with the chironomid
DCA axis 1 sample score (p < 0.005, r = 0.82 and r = -0.82, respectively) (Fig. 2).
The RDA revealed that 37.7 and 12.2 % of variance of the chironomid samples were explained
by the first and second axes respectively by all six variables while 35.2 and 10.3% of variance
were explained by three remaining significant and independent variables: MS, TN and Al (Fig.
4). Among these variables, TN alone independently explained the largest percent (33.8 %) of
variance and is the main variable dominating the RDA axis 1. RDA axis 2 was co-dominated
by MS and Al. The RDA tri-plot (Fig. 4) shows that chironomid assemblage of samples were
divided into three groups: samples from the last 40 years (the first three samples represent the
top 9 cm) were strongly controlled by RDA axis 2 (MS and Al); samples between c.1700 and
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1100 cal yr BP (sample code from 11 to 17) were co-driven by both RDA axes; the rest of the
samples were strongly controlled by RDA axis 1 (Fig. 4). The full RDA results are presented in
Table 2.
Discussion
Multi-proxy indicators for catchment erosion at Lugu Lake
Both Ti and Al are stable elements derived from terrestrial source and therefore have been
widely used as indicators for catchment erosion in lake sediment records (Croudace and
Rothwell 2015; Sergeeva 1983). In addition, magnetic susceptibility has also been shown to be
an effective proxy for soil erosion (Oldfield and Richardson 1990; Oldfield et al. 2003). In the
monsoon-influenced regions of southern China, these proxies (particularly Ti) are commonly
applied to infer the strength and intensity of monsoonal precipitation (Haug et al. 2001; Zhang
et al. 2017c). Generally speaking, Ti and Al are bonded to mineral particles (including clay and
fine silt) and during high rainfall period, there were increased washings into the lake basin thus
the increase in values of Ti, Al, fraction of clay and fine silt (while the fraction of sands is
consistent), and magnetic susceptibility was expected (Fig. 3a-c and f-g). This is evident
between c.1700 and 1100 cal yr BP of the record.
A separate study was conducted by Zhang et al (2013b) with a focus on the most recent 120
years of this site. The detailed chironomid stratigraphy and geochemical data presented for the
last century suggests that the rapid increase in human-induced landscape change, including the
change of agricultural practices and deforestation, had caused significant catchment erosion
into Lugu Lake and chironomids had responded rapidly to such change (Zhang et al. 2013b).
This is supported particularly by the increase of Ti (with a maximum of 13 mg/g) and magnetic
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susceptibility (with a maximum of 349 × 10-8 m3/kg) values in the top layers of the sediment
(i.e. since 1920s A.D.) in this record. A few paleoecological records from SW China suggest a
long history of human occupation (a minimum of 2000 years) in central Yunnan Province, for
instance, at Xingyun Lake (Chen et al. 2014a; Wu et al. 2015). However, there is limited
recorded evidence of thousands-year long human activities around lakes from further north and
higher altitude (close to 3000 m.s.l. and above) of Yunnan, from Lugu Lake (Wang et al. 2017)
or nearby sites (Chang et al. 2017; Chen et al. 2014b; Xiao et al. 2014; Zhang et al. 2017a).
Instead, evidence suggests human occupation on the Yunnan Plateau intensified in the past
250-500 cal yr BP (Wang et al. 2017; Whitmore et al. 1994). The implication is that the
increased erosion in the late Holocene (c.1700-1100 cal yr BP) at this site is most likely driven
by natural climate variability.
The different responses of chironomids to catchment soil erosion
Regional and local environmental change such as continuously increasing soil erosion can
affect the community structure of chironomids (Smol et al. 2006; Walker 1995). Such shifts
can be abrupt and these changes can be demonstrated from northern Europe (Axford et al.
2009; Langdon et al. 2010a; Taylor et al. 2017), East Africa (Eggermont and Verschuren
2003), North and South America (Araneda et al. 2013; Campbell et al. 2009), the
Mediterranean (Brisset et al. 2013) and southern China (Cao et al. 2014; Zhang et al. 2011).
The chironomid assemblage changes coincide with the change in geochemical and
sedimentological erosion indicators from the Lugu Lake record. Both the erosion events
recognized at c.1700-1100 cal yr BP and in the last century were characterised by a reduction
in Procladius and an expansion in Tanytarsus species, including the increase in Tanytarsus
mendax-type since 1970s (Zhang et al., 2013) and Tanytarsus gracilentus-type from c.1700 to
1100 cal yr BP (Fig 2). It is also observed that the concentration of chironomid head capsules
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declines from both events and this could either be due to a reduction in production (less food
source) or an increase in sedimentation rates (i.e. less chironomids per relative sample amount).
More fine sediment eroded into a lake has been demonstrated to have a substantial influence on
benthic chironomids (Höss et al. 1999; Palmer et al. 1996). The increase of clay and fine silt in
the substrate could cause a fall in oxygen availability to chironomids living at the bottom of the
lake by clogging the void space in the lakebed sediments and reducing oxygen penetration
(Nogaro et al. 2006). The Lugu Lake record was characterized by the rapid decrease in the
population of Procladius during c.1700-1100 cal yr BP and the last century. Unlike most of the
species of the subfamily Chironominae, Tanypodinae family (e.g. Procladius) does not usually
use haemoglobin for oxy-regulation (Osmulski and Leyko 1986; Ramakrishnan 2002).
Therefore, low oxygen conditions could largely affect the growth of Procladius, or cause them
to migrate to shallower parts of the lake during these periods. Conversely, select taxa from the
subfamily Chironominae, notably Tanytarsus gracilentus-type, Chironomus plumosus-type,
Dicrotendipes nervosus-type, which are well-known for having stronger ability to survive in
low oxygen concentrations by utilising haemoglobin (Brodersen et al. 2004), all increased in
their relative abundance.
It is noteworthy that Polypedilum nubeculosum-type showed an increase in abundance during
c.1700 and 1100 cal yr BP (Fig 2) but declined since the onset of the severe erosion in the
1970s (Fig 4 in Zhang et al. (2013b)), suggesting the responses of chironomids to the two
separate events are not identical. Polypedilum nubeculosum-type typically occurs in the littoral
of meso-eutrophic lakes (Brooks et al. 2007) and can be associated with macrophytes (Langdon
et al. 2010b). This suggests that the increased abundance of Polypedilum nubeculosum-type
may be a response to increased nutrients in the lake, presumably exported from the catchment
through enhanced soil erosion. The decline in head capsule abundance at the same period may
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therefore relate to a period of relatively enhanced sediment accumulation (not discernible from
the radiocarbon derived age model) (Fig. 1c), which would be consistent with an increase in
lake productivity. It is notable that this phase lasted c.600 years, but thereafter the chironomid
community returned to its pre-erosion state. In other words, it was able to recover and the lake
ecosystem showed some resilience to a persistent change over a multi-centennial timescale.
Polypedilum nubeculosum-type is also known as bottom collector-gatherers. Their reduction
during the recent high erosion period is possibly due to the loss of favourable habitats as the
substrate may be difficult to enter when there was a large amount of both fine silt/clay and
sands loaded. However during the earlier event (c.1700-1100 cal yr BP) the erosion may not
have been severe enough to cause the loss of microhabitat for collector-gathers, and instead this
taxon increased its abundance during this period maybe also due to their ability to adapt to low
oxygen conditions. We noted that the shift to the dominance of the Tanytarsus morphotypes in
the assemblage during the two erosion events may be because of they are filter-feeders and
were readily adapted to new habitats.
The RDA results (Fig. 4) demonstrate these lines of interpretation well, where samples from
c.1700-1100 cal yr BP showed a stronger relationship with RDA axis 1 (driven by lake
productivity indicators), though influenced by RDA axis 2 (driven by soil erosion indicators),
and samples from the last century are strongly dominated by RDA axis 2 (soil erosion
indicators). In summary, the evidence from c.1700-1100 cal yr BP is indicative of a climate
impact, rather than a human impact, which tend to have longer-lasting impacts on lake
ecosystems, and often result in lakes being unable to recover to their pre-impact state (Folke et
al. 2004; Scheffer et al. 2001).
Paleoclimate inferences from the Lugu Lake record of the last 2500 years
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We compared Lugu Lake record with the local and regional climate reconstructions (Fig. 5A-
C) to further reconcile the possible drivers for the c.1700-1100 cal yr BP changes as increasing
of regional rainfall was suspected. The Lugu Lake record reassembles the pollen concentration
changes (Fig. 5C) at the site and the precipitation record based on a pollen transfer function
from Xingyun Lake, in central Yunnan Province (Fig. 5B) (Chen et al. 2014a; Lu et al. 2011;
Wang et al. 2017). Moreover, we observed that the timing of the peaks of both the
reconstructed rainfall at Xingyun Lake (Fig. 5A) and chironomid indicated catchment erosion
at Lugu Lake (Fig. 5D-F) are coincident with the broad and gradual increase of tree pollen
percentages at Lugu Lake since c. 1700 cal yr BP (Fig. 5C). This confirms that wet conditions
were likely prevailed, which supported tree growth rather than forest clearance induced erosion
around the catchment during this period. These also correspond well with the inferred
strengthening or resuming of the Indian summer monsoon since c.1400-1200 cal yr BP
(Fleitmann et al. 2003), suggesting that the catchment erosion at Lugu Lake during this period
is likely due to a stronger monsoon-induced increased regional precipitation. While the trends
of these pollen-based reconstructions are similar in timing to chironomid-inferred erosion, the
match is not precise. This is however, unsurprising because usually pollen (especially trees) has
a lagged response to the change of climatic conditions while chironomids, which are aquatic
invertebrates with short life cycles responding instantaneously. We noted also that yet only a
few reconstructions (e.g. Chen et al. 2014a; Wang et al. 2017) from the region recorded the
increase of monsoonal precipitation during c.1700 and 1100 cal yr BP, thus more records are
required to elucidate a regional pattern and to further relate this change to a global scaled event.
Conclusions
A multi-proxy record with a special focus on chironomids from a deep, alpine lake, Yunnan
Province spanning the past c.2500 years was developed. The record was divided into three
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zones from c.2500 yr BP until 1920 AD based on the changes in three dominant chironomid
taxa: Procladius, Tanytarsus gracilentus-type and Polypedilum nubeculosum-type. The
chironomid variability and geochemical data from Lugu Lake together show two major
catchment erosion events that occurred during c.1700-1100 cal yr BP and the last century. Both
events change the chironomid assemblage in the record. The paleoclimate data and documented
history suggest that the erosion which occurred in the last century was caused by human
activities and it was more severe than the event of c.1700-1100 cal yr BP. We provide further
evidence that the c.1700-1100 catchment erosion at Lugu Lake is likely due to the regional
precipitation increase in southwestern China, which may be linked to the strengthening of the
Indian summer monsoon in the late Holocene. The results demonstrated that chironomids from
Lugu Lake responded strongly and instantly to catchment soil erosion and the response of some
taxa, such as Polypedilum nubeculosum-type, could be used to differentiate the strong/weaker
erosion events.
Acknowledgements
Financial support for field work and sample analyses of this work was provided through the
Program of Global Change and Mitigation (2016YFA0600502), the National Basic
Technological Research of China (2014FY110400) and the National Natural Science
Foundation of China (No. 41302277, 41572337). J. Chang also received the General Financial
Grant from the China Postdoctoral Science Foundation (No. 2017M610359). We thank X.D.
Yang, Q. Wang and Y.L. Li for field assistance.
Figure and Table captions and legends
Fig. 1. (a) Locations of Lugu Lake, southwestern China (red triangle) and the key paleorecords
discussed in the texts: Dongge Cave (1), Qunf Cave (2), Xingyun Lake (3), Tiancai Lake (4),
Heihai Lake (5), Shudu Lake (6), Wuxu Lake (7), Tengchongqinghai (TCQH) (8) and Erhai
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Lake (9). The direction of the Indian summer monsoon (ISM), East Asian summer monsoon
(EASM) are marked in orange arrows. (b) Lake bathymetry and coring site (red triangle). (c)
The age-depth model constructed based on 137Cs dating and six calibrated AMS radiocarbon
dates (blue dots) for the 95 cm sediment core of Lugu Lake.
Fig. 2. The abundance of major chironomid taxa (%) presented in the Lugu Lake record over
the past c.2500 years. Three major zones are determined based on the compositional shift in
chironomid assemblage (solid horizontal lines). Total count of head capusles (sum of hcs) and
and the DCA first axis score are shown in black solid lines in two separate panels on the right
side.
Fig. 3. The variability of selected physical and geochemical proxies plotted against the
chronology from the sediment core of Lugu Lake covering the last 2500 years: (a) and (b)
show the concentration of chemical elements Titanium (Ti) and Aluminium (Al); (c) magnetic
susceptibility (MS); (d) total organic carbon (TOC); (e) total nitrogen (TN); (f) percentage (%)
of fine silt and clay sediments with grain sizes smaller than 4 µm; (g) percentage (%) of sands
with grain sizes larger than 63 µm.
Fig. 4. RDA tri-plot with 25 chironomid samples, 13 main chironomid taxa and six selected
environmental variables (total organic carbon: TOC; total nitrogen: TN; fraction of fine silt and
clay: GS < 4 µm; concentrations of chemical elements: Al, Ti and magnetic susceptibility: MS)
included. Solid circles indicate samples from the two identified erosion events where sample
code 1-3 represent the top ~10 cm (the last century) and sample code 12-17 show samples from
65-45 cm (1700-1100 cal yr BP); open circles indicate the rest 16 samples; closed triangle
represents 13 main chironomid taxa present over c. 2500 cal yr BP in Lugu Lake sediment.
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Fig. 5. Comparison of the Lugu Lake paleoclimate proxies with the regional Asian summer
monsoon precipitation records spanning the last c.2500 years. (A) Pollen-based precipitation
reconstruction from Xingyun Lake, central Yunnan Province (Chen et al., 2014a). (B) Total
pollen concentration (Wang et al. 2017) and (C) tree pollen percentages (Wang et al. 2017) of
Lugu Lake (D) the abundance (%) of Procladius, (E) the abundance (%) of Tanytarsus
gracilentus-type and (F) chironomid DCA first axis sample score analysed from this record.
Table. 1. Description of the changes in chironomid taxa, geochemical and physical proxies
(Titanium (Ti), Aluminium (Al), magnetic susceptibility (MS), Total organic carbon (TOC),
Total nitrogen (TN) and grain sizes (GS)) throughout the three divided zones as shown in Fig 2
and 3.
Table. 2. Summary statistics for the redundancy analysis using all six environmental variables
(Titanium (Ti), Aluminium (Al), magnetic susceptibility (MS), Total organic carbon (TOC),
Total nitrogen (TN) and grain sizes (GS)), 13 main chironomid taxa and 25 samples over c.
2500 cal yr BP from Lugu Lake.
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Zhang, E., A. Bedford, R. Jones, J. Shen, S. Wang & H. Tang, 2006. A subfossil chironomid-total phosphorus inference model for lakes in the middle and lower reaches of the Yangtze River. Chinese Science Bulletin 51(17):2125-2132 doi:10.1007/s11434-006-2062-8.
Zhang, E., Y. Cao, P. Langdon, Q. Wang, J. Shen & X. Yang, 2013a. Within-lake variability of subfossil chironomid assemblage in a large, deep subtropical lake (Lugu lake, southwest China). Journal of Limnology 72(1):117-126.
Zhang, E., J. Chang, Y. Cao, W. Sun, J. Shulmeister, H. Tang, P. G. Langdon, X. Yang & J. Shen, 2017a. Holocene high-resolution quantitative summer temperature reconstruction based on subfossil chironomids from the southeast margin of the Qinghai-Tibetan Plateau. Quaternary Science Reviews 165:1-12 doi:https://doi.org/10.1016/j.quascirev.2017.04.008.
Zhang, E., J. Chang, Y. Cao, H. Tang, P. Langdon, J. Shulmeister, R. Wang, X. Yang & J. Shen, 2017b. A chironomid-based mean July temperature inference model from the south-east margin of the Tibetan Plateau, China. Clim Past 13(3):185-199 doi:10.5194/cp-13-185-2017.
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Figure 1
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Figure 2
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Figure 3
Figure 4
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Figure 5
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Zone Description of Ti, Al and TOC and Grain sizes
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and time period
chironomids magnetic susceptibility (MS)
TN
Zone 1 (95-66 cm)c.2500 – 1700 cal yr BP
High abundance of Procladius (average of 78%)
The second most abundant taxon was Polypedilum nubeculosum-type (average of 15%)
All other taxa appeared at low levels
DCA axis 1 score was relatively high ( average of 0.86)
Ti and Al showed a significant and positive correlation
They both showed a stable low level (with an average of 8 mg/g for Ti and 47 mg/g for Al)
MS values were low
TOC and TN were strongly and positively correlated
Their concertation remained relatively high and stable with an average of 10% of TOC and 1 mg/g of TN
The fractions of clay/fine silt and sands were both at low levels with an average of 9.7% and 2.5% respectively
Zone 2(62-41 cm)c.1700 – 1100 cal yr BP
A rapid decrease in Procladius (from an average of 78% to 46%)
A large increase in Polypedilum nubeculosum-type (from 15% to 30%)
A distinct increase in Tanytarsus gracilentus-type (from 1.5% to 15%)
All other taxa appeared more regularly but all with an average abundance of less than 5%.
DCA axis 1 score largely declined (from 0.86 to 0.23)
A marked increase was shown since c.1700 cal yr BP and both elements reached their peak level (11 mg/g for Ti and 62 mg/g for Al) at around 1400 cal yr BP
MS increased steadily
Both dropped rapidly since c.1700 cal yr BP
Both reached their minimum values (5.5 % of TOC and 0.51 mg/g of TN) at c.1300 cal yr BP.
The fine silt/clay fraction at c.1400 cal yr BP increased to 28% followed by a decline
The fine silt/clay fraction increased again from c.1000 cal yr BP
Changes in the fraction of sands were relatively minor
Zone 3(40-1 cm)c.1100 cal yr BP – present
Similar to Zone 1 An re-expansion of
Procladius which increased to 75%
Polypedilum nubeculosum-type dropped back to 17%, similar level to Zone 1
Tanytarsus gracilentus-type dropped back to 3%
All other taxa stayed below 5%
DCA axis 1 score recovered to similar level as zone 1 with an average of 0.68 (excluding the top 3 samples)
A gradual decrease in the concentration of both elements was followed
They both remained at stable levels (average 9 mg/g for Ti and 55 mg/g for Al) from c.1100 cal yr BP- c.1920s A.D.
Al, Ti and MS all increased to historical highest levels in the last
A slow recovery was shown since c.1100 cal yr BP for both until c.1920 AD
A rapid drop occurred in the top section of the core (i.e. the last c.100 years).
The fraction of fine silt/clay remained was at a relatively high level (average of 27%) between c.1000-580 cal yr BP
It declined with variability between 580-130 cal yr BP
Both sands and fine silt/clay fractions increased largely at the beginning of 1920s AD
28
c.100 yearsTable 1.
Table 2
Axes 1 2 3 4 Total variance Eigenvalues 0.377 0.122 0.027 0.013 1
Species-environment correlations 0.928 0.729 0.592 0.57Cumulative percentage variance of
species data 37.7 49.9 52.6 53.8 of species-environment relation 68.5 90.6 95.5 97.8
Variable Coefficients Correlations Inflation Axis 1 Axis 2 Axis 1 Axis 2 Factor
magnetic susceptibility (MS) -0.08 0.19 0.56 0.54 14.4clay and fine silt content (GS < 4 um) -0.36 0.10 0.33 0.23 1.8
TOC -1.18 0.59 -0.88 -0.08 42.9TN 0.03 0.77 -0.86 0.09 31.1Al -0.54 -0.76 0.67 0.41 24.5Ti 0.60 2.15 0.72 0.42 45.3
29
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